Synthetic aperture radar (SAR) is a high resolution radar imaging technology with significant interest in remote sensing applications. SAR exploits the motion of a radar platform, such as a satellite, a plane, a vehicle, or a boat, to synthesize a virtual array with a very large aperture that can image large areas at a high resolution.
Conventional SAR transmits pulse signals at a uniform rate. The transmitted pulses are usually linearly frequency-modulated (FM) chirps, of increasing or decreasing frequency (upchirps or downchirps, respectively). The corresponding received echoes, reflected from the region of interest, are processed to reconstruct a two-dimensional complex-valued image (i.e., the desired information is conveyed both in the magnitude and the phase of the image). The resolution of the image on the axis perpendicular to the motion of the platform (range) is determined by the bandwidth of the transmitted pulse, while the resolution along the axis of motion (azimuth) depends on the pulsing rate or pulse repetition frequency (PRF).
Conventional SAR exhibits a fundamental trade-off between the resolution of the azimuth and the length of the range imaged. This is due to the need to separate the pulse transmission from the echoes reception. Most conventional SAR systems use the same antenna for the transmission of the pulse and the reception of the received echoes. Thus, while a pulse is transmitted, the radar cannot receive the reflected echo of another pulse. Even when the antennas are separate, their proximity causes significant interference at the receiving antenna during the pulse transmission of the transmitting antenna. Thus the received signal contains minimal, if any, information from the reflected echo.
If the PRF is very high, the transmitted pulses interfere with the reception of the received echoes and cause missing data. In other words, the time interval between two transmitted pulses has to be long enough so that the reflected echoes can be fully acquired before a next pulse is transmitted.